1. Field of the Invention
The present invention relates to an exposure apparatus that uses extreme ultraviolet (“EUV”) light for exposure.
2. Description of the Related Art
The semiconductor manufacturing photolithography has recently promoted use of the exposure light having a shorter wavelength from the i-line and the g-line to the KrF excimer laser and the ArF excimer laser. As the use of the exposure light having a shorter wavelength is promoted, a finer mask pattern can be transferred on to a wafer. However, the lithography using the UV light has limits in principle of exposing a thinner critical dimension (“CD”). Accordingly, the EUV lithography calls attentions, which uses the EUV light having a wavelength of 13 to 20 nm smaller than that of the UV light.
The EUV light typically has a wavelength of 13.5 nm, and can provide a much smaller resolution than the conventional photolithography, but is likely to be absorbed into a material. Therefore, with a conventional dioptric optical system in the atmospheric pressure as in the lithography that uses the UV light for a light source, the EUV light is absorbed into a glass material and a gas in the glass material and only a very small dose can reach the substrate, such as a wafer. Therefore, it is necessary to use a catoptric optical system in a vacuum for the exposure with the EUV light. Japanese Patent Laid-Open No. 2003-45782 discloses a reduction projection exposure apparatus that uses the EUV light for the exposure light.
Degasifying arises from an actuator located in the exposure apparatus or a resist coated substrate when the inside of the EUV exposure apparatus becomes a vacuum environment. Degasifying also arises from an irradiated area on a wafer when the EUV light is irradiated onto the resist applied substrate. A component of released gas by the degasifying contains hydrocarbons, and forms a carbon layer on a surface of an optical element by a photochemical reaction with the EUV light after the hydrocarbon attaches to the surface of the optical element. A component of released gas by the degasifying also contains water, and oxidizes the surface of the optical element after the water attaches to the surface. A carbon layer formed on the optical element surface or the optical element's deteriorated surface due to the oxidation will reduce the reflective index of the optical elements. The reduction in reflective index will lower the exposure apparatus' throughput and will lead to the productivity.